The present invention is related to the field of delay elements and more specifically to a one-bit delay element for use in a differential demodulator.
In a differential demodulation apparatus, a mixer receives two IF signals, one directly and the other delayed by one bit in delay element T.sub.d as illustrated in FIG. 1. The IF signal is applied directly from hybrid H to the mixer 10, while the delayed signal is applied to the mixer 10 by way of the delay element T.sub.d. The demodulated output from mixer 10 is applied to a low pass filter 12.
The one bit delay element T.sub.d in the demodulator must be insensitive to temperature variations. The performance of the differential demodulator degrades as the number of phases in the phase shift keyed (PSK) signal increase. Thus, temperature compensation for the delay line becomes even more important at high modulation levels. Coaxial cables can be used as the delay element T.sub.d and perform well as long as temperature variations are confined to a few degrees. As the temperature variations increase the delay time changes and the bit error rate increases. For example, cable RG-188 has a temperature coefficient of 250 ppm/C..degree.. This corresponds to an approximate phase shift of 4.degree. at 70 MHz for a 40C..degree.variation. This phase shift produces a substantial degradation of the differential modulation at high modulation levels.
Further problems associated with the use of coaxial cable as a delay element are related to the size of the coaxial cable needed to provide the requisite time delay. For example, the delay in cable RG-188 amounts to approximately one nanosecond/food. For a 25 megabit data rate, 40 feet of cable are required. At a 10 megabit data rate, the amount of cable required is approximately 100 feet which is too bulky to be used.